2021 EJC Prelims Paper 2 (solution with comments)

© EJC [Turn Over EUNOIA JUNIOR COLLEGE JC2 Preliminary Examination 2021 General Certificate of Education Advanced Level Higher 2 CANDIDATE NAME CIVICS GROUP 2 0 – REGISTRATION NUMBER CHEMISTRY Paper 2 Structured Questions 9729/02 16 September 2021 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Do not use paper clips, highlighters, glue or correction fluid. Write your name, civics group, registration number on all the work you hand in. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Answer all questions on the Question Paper The number of marks is given in brackets [ ] at the end of each question or part question. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. For Examiner’s Use Paper 2 1 / 15 2 / 12 3 / 13 4 / 15 5 / 20 Total / 75 This document consists of 19 printed pages and 1 blank page.

2 © EJC 9729/02/J2PE/21 For Examiner’s Use 1 Nitrosyl chloride, NOCl decomposes at temperatures above 100 oC to form nitrogen monoxide and chlorine as shown in the equation below. 2NOCl(g) → 2NO(g) + Cl2(g) (a) A sample of NOCl was allowed to decompose at 127 ºC and 147 atm. Assuming that NOCl behaves ideally, show that its initial concentration is 4.48 mol dm−3. [2] Marker’s Comments: • Generally well done, but a number of students were not clear in showing the conversion of m −3 to dm −3. Some students thought that volume was calculated in cm −3 instead of m−3. (b) A student pointed out that the assumption in (a) that NOCl behaves ideally is not valid. Suggest and explain two possible reasons why this may be so. ................................ ................................ ................................ ................................ .. ................................ ................................ ................................ ................................ .. ................................ ................................ ................................ ................................ .. ................................ ................................ ................................ .............................. [2] Marker’s Comments: • A significant number of students stated that NOCl was a large molecule, which led to its volume becoming significant. This is not accepted as NOCl only comprises 3 atoms and is not large enough for the assertion to hold. • A number of students were still making the same mistake of stating that the volume o f the gas (instead of gas particle) is significant compared to the volume of the container, without realising that the volume of the gas is the volume of the container. • Some students tried to argue in terms of the temperature being too high/low, but 127 ºC is neither too high or too low